Radio communication system, high-power base station, low-power base station, and communication control method

ABSTRACT

A radio communication system  1  includes a pico-cell base station PeNB installed in a communication area of a macro-cell base station MeNB, having lower transmission power than the macro-cell base station MeNB, and expanded in its coverage. The macro-cell base station MeNB determines a degree of expanding the coverage of the pico-cell base station PeNB, according to an amount of usable PDSCH resources of the macro-cell base station MeNB.

TECHNICAL FIELD

The present invention relates to a radio communication system, ahigh-power base station, a low-power base station, and a communicationcontrol method, to which a heterogeneous network is applied.

BACKGROUND ART

LTE (Long Term Evolution) and LTE Advanced which is an advanced type ofLTE are next-generation systems providing communication which is fasterand supports a larger capacity than the third-generation and thethird-and-a-half generation cellular radio communication systemscurrently in operation. LTE and LTE Advanced are standardized by astandardization organization called 3GPP (3^(rd) Generation PartnershipProject).

In a downlink in the LTE systems (including LTE Advanced), a radio basestation sends user data to a radio terminal by using a data transmissionchannel called PDSCH (Physical Downlink Shared Channel). Note that thedownlink refers to communication in a direction from the radio basestation to the radio terminal, whereas an uplink refers to communicationfrom the radio terminal to the radio base station.

In LTE Advanced, provision of a heterogeneous network is underconsideration. In a heterogeneous network, low-power base stations(so-called pico-cell base stations, femto-cell base stations, or relaynodes) are installed in a communication area of a high-power basestation (a so-called macro-cell base station). A heterogeneous networkis capable of distributing the load of the high-power base station tothe low-power base stations.

However, since a radio terminal is generally connected to a radio basestation which sends radio signals with the highest received power amongmultiple radio base stations, in the heterogeneous network the radioterminal may have a low chance of being connected to a low-power basestation with low transmission power.

In consideration of such circumstances, there is proposed a method forexpanding the coverage (communication area) of the low-power basestation by performing control such that the radio terminal is connectedto the low-power base station even when the received power from thelow-power base station is not the highest (see, for example, Non-patentDocument 1).

PRIOR ART DOCUMENT Non-Patent Document

NON-PATENT DOCUMENT 1: 3GPP R1-093433 “Importance of Serving CellSelection in Heterogeneous Networks” February, 2010.

SUMMARY OF THE INVENTION

When radio resources used as data transmission channels by neighboringradio base stations overlap each other, the data transmission channel ofone of the radio base stations receives interference from the datatransmission channel of the other base station, and therefore user datacannot be received normally from the one radio base station via its datatransmission channel.

This problem is even more severe in the method of expanding the coverageof the low-power base station in the heterogeneous network since thedata transmission channel of the low-power base station is highly likelyto receive strong interference from the data transmission channel of thehigh-power base station.

Accordingly, the present invention has an objective of providing a radiocommunication system, a high-power base station, a low-power basestation, and a communication control method capable of suppressinginterference between base stations even when the coverage of thelow-power base station is expanded.

The present invention has the following features in order to solve theaforementioned problem. First of all, a feature of the radiocommunication system is summarized as follows. A radio communicationsystem comprises: a high-power base station (macro-cell base stationMeNB); and a low-power base station (e.g. pico-cell base station PeNB)installed in a communication area of the high-power base station, havinglower transmission power than the high-power base station, and expandedin its coverage, wherein the radio communication system furthercomprising a determination unit (bias value determination unit 123 orbias value determination unit 223) configured to determine a degree ofexpanding the coverage of the low-power base station, according to anamount of usable resources which are radio resources usable as aparticular downlink channel (e.g. PDSCH) by the high-power base station.Here, the particular downlink channel is a downlink data transmissionchannel (PDSCH in LTE) for example. However, the particular downlinkchannel may be a downlink control information transmission channel(PDCCH in LTE) and so on, not apply only to downlink data transmissionchannel. The low-power base station is a pico-cell base station or afemto-cell base station for example. However, the low-power base stationmay be a relay node and so on, not apply only to the pico-cell basestation or the femto-cell base station.

With the radio communication system according to the aforementionedfeature, a degree of expanding the coverage of the low-power basestation is determined according to an amount (i.e. possibility ofinterference occurrence) of usable resources which are radio resourcesusable as a particular downlink channel by the high-power base station.Thereby, the coverage of the low power base stations can be expandedappropriately considering the possibility of interference occurrence.Accordingly, interference between the base stations can be suppressedeven if the coverage of the low power base stations is expanded.

Another feature of the radio communication system is summarized asfollows. In the radio communication system according to theaforementioned feature, the determination unit determines the degree ofexpanding the coverage of the low-power base station such that thedegree becomes larger as the usable resources decrease.

Another feature of the radio communication system is summarized asfollows. In the radio communication system according to theaforementioned feature, the usable resources decrease, the determinationunit determines the degree of expanding the coverage of the low-powerbase station such that the degree is made larger than the degreedetermined before the usable resources decrease.

Another feature of the radio communication system is summarized asfollows. The radio communication system according to the aforementionedfeature further comprising: a selector (connection target selector 121,connection target selector 221) configured to select, as a connectiontarget of a radio terminal, the base station providing the highestreception quality value on the basis of a first reception quality value(RSRP_(MeNB)) indicating reception quality of a radio signal that theradio terminal receives from the high-power base station, a secondreception quality value (RSRP_(PeNB)) indicating reception quality of aradio signal that the radio terminal receives from the low-power basestation, and a correction value (bias value) for correcting the secondreception quality value to a larger value, wherein the correction valueindicates the degree of expanding the coverage of the low-power basestation, and the determination unit determines the correction valueaccording to the amount of usable resources.

Another feature of the radio communication system is summarized asfollows. In the radio communication system according to theaforementioned feature, the particular downlink channel is a datatransmission channel for transmitting user data to a radio terminal.

Another feature of the radio communication system is summarized asfollows. In the radio communication system according to theaforementioned feature, the usable resources are at least part of atotal downlink frequency band (total resource blocks).

Another feature of the radio communication system is summarized asfollows. In the radio communication system according to theaforementioned feature, the usable resources are at least part of a timerange in a total downlink time frame (subframe or radio frame).

A feature of a high-power base station is summarized as follows. Ahigh-power base station comprises: a determination unit (bias valuedetermination unit 123) configured to determine a degree of expandingcoverage of a low-power base station (e.g. pico-cell base station PeNB)according to an amount of usable resources which are radio resourcesusable as a particular downlink channel (e.g. PDSCH) by the high-powerbase station, the low-power base station installed in a communicationarea of the high-power base station and having lower transmission powerthan the high-power base station; and a transmitter (X2 interfacecommunication unit 140) configured to transmit, to the low-power basestation, information indicating the expansion degree determined by thedetermination unit.

A feature of a low-power base station is summarized as follows. Alow-power base station (e.g. pico-cell base station PeNB) installed in acommunication area of a high-power base station and having lowertransmission power than the high-power base station, comprises: adetermination unit (bias value determination unit 223) configured todetermine a degree of expanding coverage of the low-power base stationaccording to an amount of usable resources which are radio resourcesusable as a particular downlink channel by the high-power base station.

A feature of a communication control method is summarized as follows. Acommunication control method comprises: determining a degree ofexpanding coverage of a low-power base station according to an amount ofusable resources which are radio resources usable as a particulardownlink channel by a high-power base station, wherein the low-powerbase station installed in a communication area of the high-power basestation and having lower transmission power than the high-power basestation.

Another feature of a communication control method is summarized asfollows. A communication control method comprises: causing a high-powerbase station to transmit, to a low-power base station, informationindicating an amount of usable resources which are radio resourcesusable as a particular downlink channel by the high-power base station,the low-power base station installed in a communication area of thehigh-power base station and having lower transmission power than thehigh-power base station; causing the low-power base station to determinea degree of expanding coverage of the low-power base station based onthe information indicating the amount of usable resources received fromthe high-power base station, and to transmit, to the high-power basestation, information indicating the determined degree of expanding thecoverage of the low-power base station; and causing the high-power basestation to receive the information indicating the degree of expandingthe coverage of the low-power base station, which is transmitted fromthe low-power base station.

The present invention can provide a radio communication system, ahigh-power base station, a low-power base station, and a communicationcontrol method capable of suppressing interference between the basestations even when the coverage of the low-power base station isexpanded.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating the overview of an LTE system accordingto a first embodiment and a second embodiment.

FIG. 2 is a frame configuration diagram showing a frame configurationused when a FDD scheme is employed.

FIG. 3 is a schematic configuration diagram of a radio communicationsystem according to the first embodiment.

FIG. 4 is a diagram illustrating interference control according to thefirst embodiment and the second embodiment.

FIG. 5 is a block diagram showing the configuration of a macro-cell basestation according to the first embodiment.

FIG. 6 is a block diagram showing the configuration of a pico-cell basestation according to the first embodiment.

FIG. 7 is an operation sequence diagram showing the operations of theradio communication system according to the first embodiment.

FIG. 8 is a block diagram showing the configuration of a macro-cell basestation according to the second embodiment.

FIG. 9 is a block diagram showing the configuration of a pico-cell basestation according to the second embodiment.

FIG. 10 is an operation sequence diagram showing the operations of aradio communication system according to the second embodiment.

FIG. 11 is a diagram illustrating a case where PDSCH resources aredivided by time.

FIG. 12 is a diagram illustrating another case of dividing PDSCHresources by time.

DESCRIPTION OF THE EMBODIMENTS

Descriptions are given of a first embodiment, a second embodiment, andother embodiments of the present invention. In the drawings referred toby the following embodiments, the same or similar parts are given thesame or similar reference numerals.

[Overview of the LTE System]

Before describing the first embodiment and the second embodiment, anoverview of an LTE system is described, on points related to the presentinvention.

FIG. 1 is a diagram illustrating the overview of the LTE system. Asshown in FIG. 1, multiple radio base stations eNB configure E-UTRAN(Evolved-UMTS Terrestrial Radio Access Network). Each of the variousradio base stations eNB forms a cell which consists of a communicationarea where a radio terminal UE is to be provided with services.

The radio terminal UE is a radio communication device that is owned bythe user, and is also called a user device. The radio terminal UE isconfigured to connect to a radio base station eNB from which the radioterminal UE measures the highest reference signal received power (RSRP)among the multiple radio base stations eNB. Note that it is not limitedto the RSRP, and other reception quality indices, such as SNR (Signal toNoise ratio), may be used instead.

The radio base stations eNB are capable of communicating with each othervia X2 interfaces which are logical communication paths providingcommunications between the base stations. Each of the multiple radiobase stations eNB can communicate with EPC (Evolved Packet Core), ormore specifically, MME (Mobility Management Entity)/S-GW (ServingGateway), via an S1 interface.

In radio communication between each radio base station eNB and the radioterminal UE, an OFDMA (Orthogonal Frequency Division Multiple Access)scheme is employed as the multiplexing scheme for the downlink, and anSC-FDMA (Single-Carrier Frequency Division Multiple Access) scheme isemployed as the multiplexing scheme for the uplink. Further, an FDD(Frequency Division Duplex) scheme or a TDD (Time Division Duplex)scheme is employed as the duplexing scheme.

FIG. 2(a) is a frame configuration diagram showing the configuration ofa downlink radio frame used when the FDD scheme is employed. FIG. 2(b)is a frame configuration diagram showing the configuration of a downlinksubframe.

As shown in FIG. 2(a), the downlink radio frame is configured with tendownlink subframes, and each downlink subframe is configured with twodownlink slots. Each downlink subframe is 1 ms long, and each downlinkslot is 0.5 ms long. Each downlink slot contains seven OFDM symbols inthe time-axis direction (time domain), and contains multiple resourceblocks (RB) in the frequency-axis direction (frequency domain) as shownin FIG. 2(b). Each RB contains 12 sub-carriers.

As shown in FIG. 2 (b), each downlink subframe contains two successivedownlink slots. A maximum of three OFDM symbols from the top of thefirst downlink slot of each downlink subframe is a control regionconfiguring radio resources used as PDCCH (Physical Downlink ControlChannel) for transmitting control information. The control informationcorresponds to information such as uplink and downlink schedulinginformation (i.e., information on allocated radio resources).

The rest of the OFDM symbols composing the downlink subframe is a dataregion configuring radio resources used as PDSCH (Physical DownlinkShared Channel) for transmitting user data. The radio terminal UE canidentify user data transmitted via PDSCH by decoding the controlinformation transmitted via PDCCH.

First Embodiment

The first embodiment of the present invention is described next. Thefirst embodiment is described using, as an example, a heterogeneousnetwork deployment in which pico-cell base stations PeNB which arelow-power base stations (low-output base stations) are installed insidea communication area (i.e., a macro cell) of a macro-cell base stationMeNB which is a high-power base station (a high-output base station).

In the following first embodiment, descriptions are given of (1) theconfiguration of a radio communication system, (2) interference control,(3) the configuration of a macro-cell base station, (4) theconfiguration of a pico-cell base station, (5) operations of the radiocommunication system, and (6) advantageous effects of the firstembodiment.

(1) Configuration of the Radio Communication System

FIG. 3 is a diagram of the schematic configuration of a radiocommunication system 1 according to the first embodiment.

As shown in FIG. 3, the radio communication system 1 includes amacro-cell base station MeNB, a radio terminal MUE connected to themacro-cell base station MeNB, pico-cell base stations PeNB1 to PeNB3which are installed within a macro cell MC formed by the macro-cell basestation MeNB and are adjacent to the macro-cell base station MeNB, andradio terminals PUE in pico cells PC formed by the pico-cell basestations PeNB1 to PeNB3 and are connected to the pico-cell base stationsPeNB, respectively. Below, the pico-cell base stations PeNB1 to PeNB3are simply called pico-cell base stations PeNB when no particulardifferentiation is necessary among them. The macro-cell base stationMeNB and the pico-cell base stations PeNB use a common frequency band.In addition, the pico cells PC formed by the pico-cell base stationsPeNB are called “hot zones” below.

The pico-cell base stations PeNB (also called hot-zone nodes) arelow-power base stations with lower transmission power than themacro-cell base station MeNB, and are installed in high-traffic zones ofthe macro cell. In the heterogeneous network, the pico-cell basestations PeNB have low transmission power. Accordingly, when a maximumreceived power standard (called an RP standard below), which is aconnection target selection standard where the radio terminal UE selectsand connects to the radio base station eNB having the highest RSRP, isemployed, the coverage of the pico-cell base stations PeNB mightdecrease. Especially when the pico-cell base stations PeNB are locatedclose to the macro-cell base station MeNB, the coverage of the pico-cellbase stations PeNB is so decreased that the pico-cell base stations PeNBcannot be used effectively.

The following two methods can be mainly used to allow the coverage ofeach pico-cell base station PeNB to expand without increasing thetransmission power of the pico-cell base station PeNB.

In the first method, instead of using the RP standard where the radiobase station eNB transmitting radio signals of the highest RSRP isselected as a connection target of the radio terminal UE, the radio basestation eNB having the smallest propagation loss (path loss) with theradio terminal UE is selected as a connection target of the radioterminal UE. In this way, the radio base station eNB closest to theradio terminal UE is for example selected as the connection target,allowing expansion of the coverage of the pico-cell base stations PeNB.Such a connection target selection standard is referred to as a minimumpath-loss standard (called a PL standard below).

In the second method, when the radio terminal UE can receive radiosignals from each of the macro-cell base station MeNB and the pico-cellbase stations PeNB, before comparing the RSRP of the macro-cell basestation MeNB and the RSRPs of the pico-cell base stations PeNB, a biasvalue is added to each of the RSRPs of the pico-cell base stations PeNB.By giving bias to the RSRPs of the pico-cell base stations PeNB (i.e.,adding a bias value to the RSRPs of the pico-cell base stations PeNB),the RSRPs given the bias are more likely to exceed the RSRP of themacro-cell base station MeNB. Consequently, the pico-cell base stationsPeNB are preferentially selected as the connection target, achievingexpansion of the coverage of the pico-cell base stations PeNB. Such aconnection target selection standard is referred to as a range expansionstandard (called an RE standard below). By making the bias value equalto the difference in transmission power between the macro-cell basestation MeNB and the pico-cell base station PeNB (e.g., 16 dB), the REstandard becomes a connection target selection standard equivalent tothe PL standard.

In the first embodiment, the coverage of the pico-cell base station PeNBis expanded using the RE standard. For example, the connection target ofthe radio terminal UE is selected by the radio terminal UE when theradio terminal UE is in standby (an idle state), and selected by theradio base station eNB when the radio terminal UE is in communication(an active state). In the active state, the radio terminal UEperiodically gives RSRP measurement values to the radio base station eNBto which the radio terminal UE is connected. Accordingly, the radio basestation eNB to which the radio terminal UE is connected can select thenext connection target of the radio terminal UE and hand-over the radioterminal UE to the next connection target.

The macro-cell base station MeNB uses a PDSCH to transmit user data tothe radio terminal MUE. The pico-cell base station PeNB uses a PDSCH totransmit user data to the radio terminal PUE. When the frequency bandsof these PDSCHs overlap each other, the PDSCHs of the macro-cell basestation MeNB and the pico-cell base station PeNB interfere with eachother.

When the coverage of the pico-cell base station PeNB is expanded, theradio terminal PUE connected to the pico-cell base station PeNBsometimes receives higher power from the macro-cell base station MeNBthan from the pico-cell base station PeNB. In this case, the PDSCH ofthe pico-cell base station PeNB receives strong interference from thePDSCH of the macro-cell base station MeNB, making the radio terminal PUEunable to receive (decode) user data.

(2) Interference Control

In the downlink of the heterogeneous network, if the coverage isexpanded by giving bias according to the RE standard so that thecoverage may be larger than the hot zone formed by the RP standard, thedifference in transmission power between the macro-cell base stationMeNB and the pico-cell base station PeNB causes interference power to begreater than desired signal power.

Then, the radio terminal UE not having an optimal SINR is consequentlycontained in the hot zone. Such a radio terminal UE basically suffersstrong interference from the macro-cell base station MeNB having hightransmission power, so that the SINR becomes very low.

To avoid this, the following interference control is performed in thefirst embodiment. FIG. 4 is a diagram illustrating interference controlaccording to the first embodiment.

As shown in FIG. 4(a), only part of the PDSCH resources (correspondingto the data region shown in FIG. 2(b)) of the macro-cell base stationMeNB is usable, and the rest is not used. Thereby, the unused part isoffered to the radio terminal PUE having a low SINR in the hot zone.Herein, the PDSCH resources usable by the macro-cell base station MeNBare also called “usable PDSCH resources,” and the PDSCH resources notusable by the macro-cell base station MeNB are also called “unusablePDSCH resources.” In the first embodiment, the usable PDSCH resourcesare at least part of the total downlink resource blocks, and theunusable PDSCH resources are the rest of the total downlink resourceblocks, i.e., resource blocks other than the part described above. ThePDSCH resources can be divided in any manner, but due to the LTEspecifications, they are divided according to the resolution of afed-back CQI.

As shown in FIG. 4(b), the radio resources which correspond to theunusable PDSCH resources do not receive interference from the macro-cellbase station MeNB. Accordingly, the pico-cell base station PeNBallocates such interference-free PDSCH resources to the radio terminalPUE of a low SINR. To be more specific, the radio terminal PUEperiodically feeds back a measurement result of reception quality as achannel quality indicator (CQI) to the pico-cell base station PeNB, andthe pico-cell base station PeNB can allocate the interference-free PDSCHresources preferentially to the radio terminal PUE in response tofeedback of a favorable CQI for the interference-free PDSCH resources.

Alternatively, the pico-cell base station PeNB can be given theinterference-free PDSCH resources by being notified of the unusablePDSCH resources by the macro-cell base station MeNB. In this case, thepico-cell base station PeNB can allocate the interference-free PDSCHresources preferentially to the radio terminal PUE without waiting forfeedback of a favorable CQI for the interference-free PDSCH resources.In the first embodiment, the macro-cell base station MeNB notifies thepico-cell base station PeNB of the unusable PDSCH resources.

The amount of usable PDSCH resources of the macro-cell base station MeNB(herein, an “amount” includes the concept of “percentage”) is determinedaccording to, for example, a message from the pico-cell base stationPeNB or another macro-cell base station. In the LTE system, a messagefor restricting usage of the PDSCH resources can be exchanged betweenthe base stations via the X2 interfaces, and therefore the macro-cellbase station MeNB determines the amount of usable PDSCH resourcesaccording to the received message.

Alternatively, the amount of usable PDSCH resources of the macro-cellbase station MeNB is determined according to the traffic load of themacro-cell base station MeNB (e.g., the number of terminals beingconnected). More specifically, the amount of usable PDSCH resources ofthe macro-cell base station MeNB is decreased when the macro-cell basestation MeNB has low traffic load, whereas the amount of usable PDSCHresources of the macro-cell base station MeNB is increased when themacro-cell base station MeNB has high traffic load.

In the first embodiment, the macro-cell base station MeNB determines thebias value of the RE standard according to the amount of its usablePDSCH resources. Specifically, when the amount of usable PDSCH resourcesof the macro-cell base station MeNB is small, even a large bias value ofthe RE standard would not cause the radio terminal PUE connected to thepico-cell base station PeNB to easily receive interference from themacro-cell base station MeNB. On the other hand, when the amount ofusable PDSCH resources of the macro-cell base station MeNB is large, alarge bias value of RE standard would cause the radio terminal PUEconnected to the pico-cell base station PeNB to easily receiveinterference from the macro-cell base station MeNB. Accordingly, whenthe usable PDSCH resources of the macro-cell base station MeNB decrease,the macro-cell base station MeNB correspondingly increases the biasvalue of the RE standard, and when the usable PDSCH resources of themacro-cell base station MeNB increase, the macro-cell base station MeNBcorrespondingly decreases the bias value of the RE standard. In thisway, even when the coverage of the pico-cell base station PeNB isexpanded, interference between the PDSCHs of the base stations can besuppressed. Further, when the usable PDSCH resources of the macro-cellbase station MeNB are updated as appropriate, it is desirable that thebias value of the RE standard be newly set according to the update ofthe usable PDSCH resources of the macro-cell base station MeNB.

(3) Configuration of the Macro-Cell Base Station

The configuration of the macro-cell base station MeNB is described next.FIG. 5 is a block diagram showing the configuration of the macro-cellbase station MeNB according to the first embodiment.

As shown in FIG. 5, the macro-cell base station MeNB includes an antennaunit 101, a radio communication unit 110, a controller 120, a storageunit 130, and an X2 interface communication unit 140.

The radio communication unit 110 is configured with, for example, aradio frequency (RF) circuit, a base band (BB) circuit, or the like, andconfigured to exchange radio signals with the radio terminal PUE via theantenna unit 101. The radio communication unit 110 is also configured tomodulate transmission signals and demodulate received signals.

The controller 120 is configured with, for example, a CPU, and isconfigured to control various functions of the macro-cell base stationMeNB. The storage unit 130 is configured with a memory for example, andis configured to store various pieces of information used for thecontrol of the macro-cell base station MeNB, or the like. The X2interface communication unit 140 is configured to use an X2 interface toperform communications with other radio base stations.

The controller 120 includes a connection target selector 121, a usableresource determination unit 122, a bias value determination unit 123,and a resource allocator 124.

The connection target selector 121 is configured to select the radiobase station to which the radio terminal MUE is to be connected next,based on RSRP information (i.e., a measurement report) informed of bythe radio terminal MUE. When the radio terminal MUE receives referencesignals from the macro-cell base station MeNB and the pico-cell basestation PeNB, before comparing RSRP_(MeNB) of the macro-cell basestation MeNB and RSRP_(PeNB) of the pico-cell base station PeNB, theconnection target selector 121 gives bias to RSRPPeNB. When RSRPPeNBthus given bias is higher than RSRPMeNB, the connection target selector121 performs hand-over control, switching the connection target of theradio terminal MUE to the pico-cell base station PeNB.

The usable resource determination unit 122 is configured to determineusable PDSCH resources according to messages from other radio basestations, which messages are for limiting use of PDSCH resources andreceived by the X2 interface communication unit 140. Alternatively, theusable resource determination unit 122 is configured to determine theusable PDSCH resources according to the traffic load of the macro-cellbase station MeNB (e.g., the number of terminals being connected).

The bias value determination unit 123 is configured to determine a biasvalue of the RE standard for each of the pico-cell base stations PeNBaccording to the amount of usable PDSCH resources determined by theusable resource determination unit 122. To be more specific, when theusable PDSCH resources of the macro-cell base station MeNB decrease, thebias value determination unit 123 correspondingly increases the biasvalue of the RE standard, and when usable PDSCH resources of themacro-cell base station MeNB increase, the bias value determination unit123 correspondingly decreases the bias value of the RE standard.

The bias value of the RE standard determined by the bias valuedetermination unit 123 may be the same for all the pico-cell basestations PeNB, or may be different for each of the pico-cell basestations PeNB. For example, a relatively-large bias value of the REstandard is set for the pico-cell base station PeNB which is locatedclose to the macro-cell base station MeNB (or has small path loss) andtherefore is likely to be affected by the interference. Conversely, arelatively-small bias value of the RE standard is set for the pico-cellbase station PeNB which is away from the macro-cell base station MeNB(or has large path loss) and therefore is unlikely to be affected by theinterference.

The resource allocator 124 is configured to allocate radio resources(resource blocks) to the radio terminal MUE from the usable PDSCHresources determined by the usable resource determination unit 122. Forexample, the resource allocator 124 allocates radio resources (resourceblocks) to the radio terminal MUE from the usable PDSCH resources basedon the CQIs fed back from the radio terminal MUE and by using ascheduling algorithm such as proportional fairness (PF).

(4) Configuration of the Pico-Cell Base Station

The configuration of the pico-cell base station PeNB is described next.FIG. 6 is a block diagram showing the configuration of the pico-cellbase station PeNB according to the first embodiment.

As shown in FIG. 6, the pico-cell base station PeNB includes an antennaunit 201, a radio communication unit 210, a controller 220, a storageunit 230, and an X2 interface communication unit 240.

The radio communication unit 110 is configured with, for example, aradio frequency (RF) circuit, a base band (BB) circuit, or the like, andconfigured to exchange radio signals with the radio terminal PUE via theantenna unit 201. The radio communication unit 210 is also configured tomodulate transmission signals and demodulate received signals.

The controller 220 is configured with, for example, a CPU, and isconfigured to control various functions of the pico-cell base stationPeNB. The storage unit 230 is configured with a memory for example, andis configured to store various pieces of information used for thecontrol of the pico-cell base station PeNB, or the like. The X2interface communication unit 240 is configured to use the X2 interfaceto perform communications with other radio base stations.

The controller 220 includes a connection target selector 221 and aresource allocator 222.

The connection target selector 221 is configured to select the radiobase station to which the radio terminal PUE is to be connected next,based on the RSRPs informed of by the radio terminal PUE connected tothe pico-cell base station PeNB. When the radio terminal PUE receivesreference signals from the macro-cell base station MeNB and thepico-cell base stations PeNB, before comparing the RSRPMeNB of themacro-cell base station MeNB and the RSRPPeNB of each of the pico-cellbase stations PeNB, the connection target selector 221 gives bias to theRSRPPeNB. When the RSRPPeNB thus given bias is lower than the RSRPMeNB,the connection target selector 221 performs hand-over control, switchingthe connection target of the radio terminal PUE to the macro-cell basestation MeNB.

The resource allocator 222 is configured to allocate radio resources(resource blocks) to the radio terminal PUE. For example, the resourceallocator 222 allocates radio resources (resource blocks) to the radioterminal MUE from the usable PDSCH resources, based on the CQIs fed backfrom the radio terminal PUE and by using a scheduling algorithm such asproportional fairness (PF). In a case where the unusable PDSCH resourcesare notified of by the macro-cell base station MeNB, the resourceallocator 222 allocates interference-free PDSCH resources whichcorrespond to the unusable PDSCH resources (see FIG. 4) preferentiallyto the radio terminal PUE without waiting for feedback of a favorableCQI for the interference-free PDSCH resources.

(5) Operations of the Radio Communication System

FIG. 7 is an operation sequence diagram showing the operations of theradio communication system 1 according to the first embodiment.

In Step S11, the usable resource determination unit 122 of themacro-cell base station MeNB determines its usable PDSCH resources.

In Step S12, the bias value determination unit 123 of the macro-cellbase station MeNB determines a bias value of the RE standard for each ofthe pico-cell base stations PeNB according to the amount of usable PDSCHresources determined by the usable resource determination unit 122, andstores the bias values in the storage unit 130. The bias values storedin the storage unit 130 are thereafter referred to by the connectiontarget selector 121.

In Step S13, the X2 interface communication unit 140 of the macro-cellbase station MeNB sends the pico-cell base stations PeNB informationindicating the bias values determined by the bias value determinationunit 123 and information indicating the unusable PDSCH resourcesdetermined by the usable resource determination unit 122. The X2interface communication unit 240 of each pico-cell base station PeNBreceives the information indicating the bias values and the informationindicating the unusable PDSCH resources.

In Step S14, the resource allocator 124 of the macro-cell base stationMeNB allocates radio resources (resource blocks) to the radio terminalMUE from the usable PDSCH resources determined by the usable resourcedetermination unit 122.

In Step S15, the storage unit of 230 of each pico-cell base station PeNBstores the information indicating the bias values received by the X2interface communication unit 240. The bias values are thereafterreferred to by the connection target selector 221.

In Step S16, the resource allocator 222 of each pi co-cell base stationPeNB allocates radio resources (resource blocks) to the radio terminalPUE. Based on the information indicating the unusable PDSCH resourcesreceived by the X2 interface communication unit 240, the resourceallocator 222 allocates interference-free PDSCH resources correspondingto the unusable PDSCH resources (see FIG. 4) preferentially to the radioterminal PUE of a low SINR.

(6) Advantageous Effects of First Embodiment

As described above, according to the amount of its usable PDSCHresources, the macro-cell base station MeNB determines the bias valuesindicating the degree of expanding the coverage of the pico-cell basestations PeNB. Thereby, the coverage of the pico-cell base stations PeNBcan be expanded appropriately considering the high possibility ofinterference occurrence. Accordingly, interference between the basestations can be suppressed even if the coverage of the pico-cell basestations PeNB is expanded.

Second Embodiment

In the first embodiment, the macro-cell base station MeNB determines thebias values, but in the second embodiment, each pico-cell base stationPeNB determines its bias value. In the following, points different fromthe first embodiment are described, while overlapping points are notdescribed again.

FIG. 8 is a block diagram showing the configuration of a macro-cell basestation MeNB according to the second embodiment. As shown in FIG. 8, themacro-cell base station MeNB according to the second embodiment does nothave the bias value determination unit 123 described in the firstembodiment.

FIG. 9 is a block diagram showing the configuration of each pico-cellbase station PeNB according to the second embodiment. As shown in FIG.9, the pico-cell base station PeNB according to the second embodimentincludes a bias value determination unit 223. The bias valuedetermination unit 223 is configured to determine a bias value of the REstandard. The method of determining the bias value is the same as in thefirst embodiment.

FIG. 10 is an operation sequence diagram showing the operations of aradio communication system 1 according to the second embodiment. FIG. 10shows an example of an operation sequence performed between onepico-cell base station PeNB and the macro-cell base station MeNB.

In Step S21, the usable resource determination unit 122 of themacro-cell base station MeNB determines the usable PDSCH resources ofthe macro-cell base station MeNB.

In Step S22, the X2 interface communication unit 140 of the macro-cellbase station MeNB sends the pico-cell base station PeNB informationindicating the usable PDSCH resources (or its amount) determined by theusable resource determination unit 122. The X2 interface communicationunit 240 of the pico-cell base station PeNB receives the informationindicating the usable PDSCH resources (or its amount) of the macro-cellbase station MeNB.

In Step S23, based on the information received by the X2 interfacecommunication unit 240, the bias value determination unit 223 of thepico-cell base station PeNB determines a bias value of the RE standardfor the pico-cell base station PeNB according to the amount of usablePDSCH resources of the macro-cell base station MeNB. The bias valuedetermination unit 223 then stores the bias value in the storage unit230. The bias value stored in the storage unit 230 is thereafterreferred to by the connection target selector 221.

In Step S24, the X2 interface communication unit 240 of the pico-cellbase station PeNB sends the macro-cell base station MeNB informationindicating the bias value determined by the bias value determinationunit 223. The X2 interface communication unit 140 of the macro-cell basestation MeNB receives the information indicating the bias value.

In Step S25, the resource allocator 222 of the pico-cell base stationPeNB allocates radio resources (resource blocks) to the radio terminalPUE. The resource allocator 222 allocates interference-free PDSCHresources corresponding to the unusable PDSCH resources of themacro-cell base station MeNB (see FIG. 4) preferentially to the radioterminal PUE with a low SINR.

In Step S26, the storage unit 130 of the macro-cell base station MeNBstores the information indicating the bias value received by the X2interface communication unit 140. The bias value thus stored isthereafter referred to by the connection selector 121.

In Step S27, based on the information indicating unusable PDSCHresources received by the X2 interface communication unit 140, theresource allocator 124 of the macro-cell base station MeNB allocatesradio resources (resource blocks) to the radio terminal MUE from theusable PDSCH resources.

As described, the second embodiment can offer advantageous effectssimilar to those in the first embodiment.

Other Embodiments

As described above, the present invention has been disclosed by usingthe embodiments. However, it should not be understood that thedescription and drawings which constitute part of this disclosure limitthe present invention. From this disclosure, various alternativeembodiments, examples, and operation techniques will be easily found bythose skilled in the art.

In the embodiments described above, the PDSCH resources are divided byfrequency, but the PDSCH resources may be divided by time. FIG. 11 is adiagram showing a case of dividing the PDSCH resources by time. Any unitcan be set for the time division, but due to the LTE specifications, theresources are divided in a unit of OFDM symbol. Instead of time-dividinga subframe in a unit of OFDM symbol, a radio frame shown in FIG. 2 maybe divided by time in a unit of subframe. FIG. 12 shows a case ofdividing a radio frame by time in a unit of subframe.

In the embodiments described above, the resource division involvesdivision of PDSCH resources (i.e., division of a data region). However,the present invention is not limited to PDSCH, and also applicable todivision of PDCCH resources (i.e., division of a control region). Eitherof the frequency division or the time division may be used for thedivision of PDCCH resources.

In LTE Advanced, a relay node which is a radio base station configuringa wireless backhaul is expected to be employed, and the X2 interface isto be employed for the relay node as well. Accordingly, the relay nodecan be the low-power base station according to the present invention.

Further, the present invention is applied to the LTE system in theembodiments described above, but may be applied to other radiocommunication systems such as a radio communication system based onWiMAX (IEEE 802.16).

As described above, the present invention naturally includes variousembodiments which are not described herein. Accordingly, the technicalscope of the present invention should be determined only by the mattersto define the invention in the scope of claims regarded as appropriatebased on the description.

This application claims the benefit of priority from Japanese PatentApplication No. 2010-95546 (filed on Apr. 16, 2010), the entire contentsof which are incorporated herein by reference.

INDUSTRIAL APPLICABILITY

As described, the radio communication system, the high-power basestation, the low-power base station, and the communication controlmethod according to the present invention are capable of suppressinginterference between the base stations even when the coverage of thelow-power base station is expanded, and therefore are useful in radiocommunication such as mobile communication.

The invention claimed is:
 1. A first base station comprising: acontroller that manages an aggressor cell that causes stronginterference to a victim cell managed by a second base station, whereindownlink radio resources of the aggressor cell include control regionsand data regions that are provided in time division manner, the controlregions used for transmitting downlink control channels, the dataregions used for transmitting downlink data, and performs a time domaininterference control of setting at least one portion of the data regionsas unusable resources, so as to suppress interference caused by theaggressor cell to the victim cell, wherein the controller does not setthe control regions as the unusable resources; and a transmitter thattransmits resource information indicating time resources correspondingto the unusable resources, to the second base station via an X2interface between the first and the second base stations, wherein thevictim cell utilizes the time resources corresponding to the unusableresources of the aggressor cell using the resource information.
 2. Asecond base station comprising: a controller that manages a victim cellthat receives strong interference from an aggressor cell managed by afirst base station, wherein downlink radio resources of the aggressorcell include control regions and data regions that are provided in timedivision manner, the control regions used for transmitting downlinkcontrol channels, the data regions used for transmitting downlink data;and a receiver that receives, from the first base station via an X2interface between the first and the second base stations, resourceinformation indicating time resources corresponding to unusableresources of the aggressor cell in a case where a time domaininterference control is performed, wherein the unusable resources are atleast one portion of the data regions of the aggressor cell and do notinclude control regions, and the victim cell utilizes the time resourcescorresponding to the unusable resources of the aggressor cell.